Structure of Telephone System

Hello, friends today we are discussing the structure of the telephone system. before that, it's good to see the telephone system.

The telephone System

A telephone exchange or telephone switch is a telecommunications system used in the public switched telephone network or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers. Telecommunication is the exchange of signs, signals, messages, words, writings, images, and sounds or information of any nature by wire, radio, optical, or other electromagnetic systems.  Telecommunication occurs when the exchange of information between communication participants includes the use of technology. It is transmitted through a transmission medium, such as over physical media, for example, over electrical cable, or via electromagnetic radiation through space such as radio or light. Such transmission paths are often divided into communication channels which afford the advantages of multiplexing

History of Telephone

When Alexander Graham Bell patented the telephone in 1876, there was an enormous demand for his new invention. People purchase telephones which came in pairs.
If a telephone owner wanted to talk to another telephone owner, separate wires had to be strung to all n houses, and the cities were covered with wires passing over houses and trees in a wild jumble as shown in figure (a). That was not going to work.

 Bell formed the Bell telephone company, which opened its first switching office in 1878. The company ran a wire to each customer’s house or office. To make calls, the customer first ringing sound in the telephone company office to attract the attention of an operator, who would then manually connect the caller to the callee using a jumper cable. The model of a single switching office is in figure (b).
Then people wanted to make long-distance calls between cities, so the bell system began to connect the switching offices. The original problem soon returned to connecting every switching office to every other switching office with the wire, again it became unmanageable,  so multiple second-level offices were needed as shown in figure (c). The hierarchy grew to five levels.

a) Fully interconnected network. (b)  Centralized switch. (c) Two level hierarchy

Structure 

At present, the telephone system is organized as a highly redundant, multilevel hierarchy. Each telephone has two copper wires, that go directly to the telephone company’s nearest end office (also called a local central office). The distance is typically 1 to 10 Km. (smaller cities)
The two wire connections between each subscriber’s telephone and the end office are known as the Local Loop.
If the caller and callee both are attached to a given end office, the switching mechanism is within the office and set a direct electrical connection between the two local loops.
If the called telephone is attached to another end office, a different procedure has to be used. Each end office has a number of outgoing lines to one or more nearby switching centers, called Toll offices (or if they are within the same local area called Tandem offices). And these lines are called Toll connecting trunks.

If both the caller’s and callee’s end offices are attached to the same toll office, the connection may be established within the toll office. But if they are not in the same toll office, the path will have to be established somewhere higher up in the hierarchy, via high bandwidth called intertoll trunks or interoffice trunks. Figure (c) shows how a medium-distance connection might be routed.

Typical Circuit Route For A Medium-Distance Call.

A variety of transmission media are used for telecommunication. Local loops consist of twisted pairs. Co-axial cables, Microwave Transmission, and fiber optics are widely used between switching offices.
Analog signals are passed throughout the telephone system with the actual voice signal being transmitted as an electrical voltage from source to destination.
With the advent of digital electronics and computers, digital signaling has become possible.

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